A worker on the royal estate in Sandringham in Norfolk is believed to have been underwater "for a number of minutes" after the grasscutter he was riding went into a lake.
The man, in his 30s, had apparently become trapped underwater by the grasscutter. He was taken to Addenbrooke’s Hospital by air ambulance where he remains in a critical condition.
Emergency services were called to the scene at Lower Lake, close to the B1440 at around 1.30pm on Monday.
Two police officers and a colleague from the estate retrieved the man from the water and began CPR before paramedics arrived.
Norfolk Police and the Health and Safety Executive are continuing to investigate the accident.
A spokesman for Norfolk Police said:
A worker is in a critical condition after being pulled from a lake on the Queen's Sandringham estate.
Reports suggest that an estate worker, believed to be in his 30s, was operating a ride-on lawn mower when he fell into the lower lake yesterday.
Emergency services were called to the Norfolk retreat at around 1.30pm.
A spokesman for Norfolk Police said: "The man was retrieved from the water and was taken to Addenbrooke's Hospital by air ambulance for treatment. He is currently described as being in a critical condition."
Two men have been arrested on suspicion of murdering a teenager whose body was found on the Queen's Sandringham estate.
Norfolk Police said they had made the arrest earlier today in connection with human remains, identified as those of missing Latvian 17-year-old Alisa Dmitrijeva, which were found on the estate in Norfolk on January 1.
The men, aged 28 and 31 and from the Wisbech area, are being questioned at a police station in King's Lynn.